Ceramic metal halide lamps for general illumination utilize translucent polycrystalline alumina (PCA) discharge vessels. PCA is translucent, not transparent, due to birefringence of the hexagonal alumina grains. Because of the lack of transparency, a PCA discharge vessel is generally not suitable for focused-beam, short-arc lamps such as projection lamps and automotive headlights. For focused-beam lamps, a transparent ceramic like sapphire is required.
Aluminum oxynitride (AlON) is a transparent ceramic material with in-line transmittance values as high as that of sapphire. AlON has a cubic spinel structure and a composition that may be generally represented by the empirical formula Al(64+x)/3O32-xNx where 2.75≦x≦5. The mechanical strength and thermal expansion of AlON are close to those of PCA, so that AlON should be able to survive the stresses in high-intensity discharge (HID) lamps. In fact, several sources have identified AlON as a material suitable for HID lamps, for example, Japanese Patent No. 09-92206 and U.S. Pat. Nos. 5,924,904 and 5,231,062.
However, there remain a number of technical difficulties which must be overcome for AlON to be considered as a reliable material for HID lamps. One in particular is the reaction of AlON with the glass/ceramic frit materials used to seal the discharge vessels. In a typical HID lamp, the function of the frit is to hermetically seal the ceramic body of the discharge vessel to the feedthrough portion of the electrode assembly. The reaction of the AlON with the frit results in the formation of gas bubbles in the frit that may degrade the quality and function of the hermetic seal, particularly when higher pressures are present in the discharge vessel. Thus, it would be an advantage to be able control or eliminate the formation of these bubbles.